Pulsed laser sources

ABSTRACT

Various embodiments include modelocked fiber laser resonators that may be coupled with optical amplifiers. An isolator may separate the laser resonator from the amplifier, although certain embodiments exclude such an isolator. A reflective optical element on one end of the resonator having a relatively low reflectivity may be employed to couple light from the laser resonator to the amplifier. Enhanced pulse-width control may be provided with concatenated sections of both polarization-maintaining and non-polarization-maintaining fibers. Apodized fiber Bragg gratings and integrated fiber polarizers may be also be included in the laser cavity to assist in linearly polarizing the output of the cavity. Very short pulses with a large optical bandwidth may be obtained by matching the dispersion value of the fiber Bragg grating to the inverse of the dispersion of the intra-cavity fiber. Frequency comb sources may be constructed from such modelocked fiber oscillators. In various exemplary embodiments, low dispersion and an in-line interferometer that provides feedback, assist in controlling the frequency components output from the comb source.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.11/372,859 filed Mar. 10, 2006 and entitled “PULSED LASER SOURCES,”which is hereby incorporated by reference in its entirety; U.S. patentapplication Ser. No. 11/372,859 is a divisional of U.S. patentapplication Ser. No. 10/814,502 filed Mar. 31, 2004 and entitled “PULSEDLASER SOURCES,” now U.S. Pat. No. 7,190,705, which is herebyincorporated by reference in its entirety; U.S. patent application Ser.No. 10/814,502 also claims the benefit of U.S. Provisional PatentApplication No. 60/519,447 filed Nov. 12, 2003 entitled “POLARIZATIONMAINTAINING DISPERSION CONTROLLED FIBER LASER SOURCE OF ULTRASHORTPULSES,” which is incorporated herein by reference in its entirety. U.S.patent application Ser. No. 11/372,859 is also a continuation-in-part ofU.S. patent application Ser. No. 10/627,069, filed Jul. 25, 2003,entitled “POLARIZATION MAINTAINING DISPERSION CONTROLLED FIBER LASERSOURCE OF ULTRASHORT PULSES,” published as U.S. Patent ApplicationPublication No. 2005/0018714 A1, now U.S. Pat. No. 7,088,756, which ishereby incorporated by reference herein in its entirety.

BACKGROUND

1. Field

The present teachings relate to modelocked fiber lasers and amplifiers,such as for example, ultra-compact high power integrated fiber laserswith pulse width controlled and with concatenated sections ofpolarization maintaining fiber, as well as potential applications suchas precision metrology.

2. Description of the Related Art

Modelocked fiber lasers offer advantages over traditional solid-statelasers for ultrafast optic applications. Modelocked fiber lasers canpotentially be packaged in very small spaces and may also exhibitsuperior mechanical and thermal stability; see for example Femtolite™Series made available by IMRA™ America, Inc, Ann Arbor, Mich. Inparticular, passively modelocked fiber lasers may allow compact designsbecause of the absence of bulky optical modulators.

To compete on an equal level with modelocked solid state lasers inultrafast optics applications, however, modelocked fiber laserspreferably include the following features: 1) the output polarizationstate should preferably be well defined, 2) the construction of thefiber laser should preferably be adaptable to mass production, 3) therequired optical elements should preferably be inexpensive, 4) thedesign concept should preferably comprise saturable absorbers with wellcontrollable parameters and 5) implementing pulse amplificationpreferably be simple. These factors are important in the design ofmodelocked fiber lasers, and there is an ongoing need for improvementsin such devices.

SUMMARY

One embodiment of the present invention comprises a master oscillatorpower amplifier comprising a mode-locked fiber oscillator and a fiberamplifier. The mode-locked fiber oscillator comprises a pair ofreflective optical elements that form an optical resonator. At least oneof the reflective optical elements is partially transmissive and has areflection coefficient that is less than about 60%. The mode-lockedfiber oscillator outputs a plurality of optical pulses. The fiberamplifier is optically connected to the mode-locked fiber oscillatorthrough a bi-directional optical connection such that light from themode-locked fiber oscillator can propagate to the fiber amplifier andlight from the fiber amplifier can propagate to the mode-locked fiberoscillator.

Another embodiment of the present invention comprises a method ofproducing laser pulses. In this method, optical energy is propagatedback and forth through a gain fiber by reflecting light from a pair ofreflective elements on opposite ends of the gain fiber. Less than about60% of the light in the gain fiber is reflected back into the gain fiberby one of the reflectors. The pair of reflective elements together forma resonant cavity that supports a plurality of resonant optical modes.The resonant optical modes are substantially mode-locking to produce atrain of pulses. The train of optical pulses is propagated from thelaser cavity through one of the reflectors to a fiber amplifier along abi-directional optical path from the laser cavity to the fiber amplifierwhere the laser pulses are amplified.

Another embodiment of the present invention comprises a fiber-basedmaster oscillator power amplifier comprising a mode-locked fiberoscillator, a fiber amplifier comprising a gain fiber, andbi-directional optical path between the mode-locked fiber oscillator andthe fiber amplifier. The mode-locked fiber oscillator comprises aresonant cavity and a gain medium. The mode-locked fiber oscillatorproduces a plurality of optical pulses. The bi-directional optical pathbetween the mode-locked fiber oscillator and the fiber amplifier permitslight from the mode-locked fiber oscillator to propagate to the fiberamplifier and light from the fiber amplifier to propagate to themode-locked fiber oscillator. The mode-locked fiber oscillator comprisesa first segment of fiber and the fiber amplifier comprise a secondsegment of optical fiber. The first and second segments form asubstantially continuous length of optical fiber. In some embodiments,the first and second segments are spliced together. The first and secondsegments may be fusion spliced. The first and second segments may alsobe butt coupled together with or without a small gap, such as a smallair gap, between the first and second segments.

Another embodiment of the present invention comprises a method ofproducing laser pulses comprising substantially mode-lockinglongitudinal modes of a laser cavity to produce laser pulses andpropagating the laser pulses from the laser cavity to a fiber amplifier.The laser pulses are amplified in the fiber amplifier. Amplifiedspontaneous emission emitted from the fiber amplifier is received at thelaser cavity. A first portion of the spontaneous emission enters thelaser cavity. A second portion of the amplified spontaneous emissionfrom the laser cavity is retro-reflecting back to the fiber amplifier tocause the second portion to be directed away from the cavity toward thefiber amplifier.

Another embodiment of the present invention comprises a fiber masteroscillator power amplifier comprising a mode-locked fiber oscillator anda fiber amplifier. The mode-locked fiber oscillator comprises a firstportion of optical fiber and a pair of reflectors spaced apart to form afiber optic resonator in the first fiber portion. At least one of thefiber reflectors comprises a partially transmissive fiber reflector. Themode-locked fiber oscillator outputs a plurality of optical pulses. Thefiber amplifier comprises a second portion of optical fiber opticallyconnected to the partially transmissive fiber reflector to receive theoptical pulses from the mode-locked oscillator. The second portion ofoptical fiber has gain to amplify the optical pulses. The first portionof optical fiber, the partially transmissive fiber reflector, and thesecond portion of optical fiber comprise a continuous path formed byoptical fiber uninterrupted by non-fiber optical components.

Another embodiment of the present invention comprises a masteroscillator power amplifier comprising a mode-locked fiber oscillator anda fiber amplifier. The mode-locked fiber oscillator comprises a pair ofreflective optical elements that form an optical resonator. At least oneof the reflective optical elements comprises a partially transmissiveBragg fiber grating having a reflection coefficient that is less thanabout 60%. The mode-locked fiber oscillator outputs a plurality ofoptical pulses. A fiber amplifier is optically connected to theoscillator through an optical connection to the partially transmissiveBragg fiber grating.

Another embodiment of the present invention comprises a masteroscillator power amplifier comprising a mode-locked fiber oscillator, afiber amplifier, and a pump source. The mode-locked fiber oscillatorcomprises a pair of reflective optical elements that form an opticalresonator. At least one of the reflective optical elements is partiallytransmissive and has a reflection coefficient that is less than about60%. The mode-locked fiber oscillator outputs a plurality of opticalpulses. A fiber amplifier is optically connected to the oscillatorthrough an optical connection to the at least one partially transmissivereflective optical elements. The pump source is optically connected tothe mode-locked fiber oscillator and the fiber amplifier to pump themode-locked fiber oscillator and the fiber amplifier.

Another embodiment of the present invention comprises a frequency combsource comprising a mode-locked fiber oscillator, a non-linear opticalelement, an interferometer, and an optical detector. The mode-lockedfiber oscillator comprises a resonant Fabry-Perot optical cavity havinga cavity length, L. The mode-locked fiber oscillator outputs opticalpulses and corresponding frequency components separated by a frequencyspacing, f_(rep) and offset from a reference frequency by a frequencyoffset, f_(ceo). The non-linear optical element is positioned to receivethe optical pulses. The non-linear optical element has sufficientoptical non-linearity to generate additional frequency components thattogether with the plurality of frequency components output by themode-locked oscillator form a first set of frequencies separated by thefrequency spacing, f_(rep) and offset from the reference frequency bythe frequency offset, f_(ceo). The interferometer is optically coupledto receive the first set of frequencies. The interferometer comprises afrequency shifter that receives the first set of frequencies and thatsuperimposes a second set of frequencies on the first set of frequenciesreceived by the frequency shifter. The second set of frequenciesinterfere with the first set of frequencies to produce beat frequencies.The optical detector optically receives the beat frequencies and has anoutput for outputting the beat frequencies.

Another embodiment of the present invention comprises a method ofproducing a frequency comb. In this method, longitudinal modes of afiber laser cavity are substantially mode-locked to produce laserpulses. The laser pulses are propagate through a non-linear opticalelement to produce a first plurality of frequency components offset froma reference frequency by frequency offset, f_(ceo). The laser pulses arepropagated along an optical path that leads to an optical detector. Asecond plurality of frequency components are generated from the firstplurality of frequency components and the first and second plurality offrequency components are propagated on the optical path leading to theoptical detector. The first plurality of optical components areinterfered with the second set of optical components along the opticalpath to the optical detector to produce at least one beat frequency. Theat least one beat frequency is used to stabilize the offset frequency,f_(ceo).

Another embodiment of the present invention comprises a frequency combsource comprising a mode-locked fiber oscillator, a substantiallynon-linear optical element, an interferometer, and an optical detector.The mode-locked fiber oscillator comprises an optical fiber and a pairof reflective optical elements that form an optical cavity that supportsa plurality of optical modes. The mode-locked fiber oscillatormode-locks the optical modes to produce optical pulses and frequencycomponents having a frequency spacing, f_(rep), and offset from areference frequency by a frequency offset, f_(ceo). The substantiallynon-linear optical element is disposed to receive the optical pulses.The substantially non-linear optical element has sufficient opticalnon-linearity to generate additional frequency components that togetherwith the frequency components output from the mode-locked fiberoscillator form a first plurality of frequency components spaced by thefrequency spacing, f_(rep), and offset from the reference frequency bythe frequency offset, f_(ceo). The interferometer interferes a secondplurality of optical frequency components with the first plurality offrequency components thereby producing beat frequencies. An opticaldetector is optically connected to the interferometer to detect the beatfrequencies. The optical detector has an output that outputs the beatfrequencies.

Another embodiment of the present invention comprises a frequency combsource comprising a mode-locked fiber oscillator and a substantiallynon-linear optical element. The mode-locked fiber oscillator has aresonant cavity comprising an optical fiber having a length, L. Theresonant cavity supports a plurality of optical modes. The mode-lockedfiber oscillator mode-locks the plurality of optical modes to produce amode-locked optical signal comprising frequency components separated bya frequency spacing, f_(rep) and offset from a reference frequency by afrequency offset, f_(ceo). The substantially non-linear optical elementis positioned to receive the mode-locked optical signal. Thesubstantially non-linear optical element has sufficient opticalnon-linearity to generate additional frequency components that togetherwith the plurality of frequency components output by the mode-lockedoscillator form a first set of frequencies separated by the frequencyspacing, f_(rep) and offset from the reference frequency by thefrequency offset, f_(ceo).

Another embodiment of the present invention comprises a method ofreducing frequency noise of a frequency comb produced by a fiber-basedfrequency comb source comprising a mode-locked fiber oscillator havingan optical cavity comprising an optical fiber having a length, L. Themethod comprises reducing the dispersion in the mode-locked fiberoscillator to less than about 10,000 femtosec²/m×L.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A illustrates a schematic diagram of a fiber master oscillatorpower amplifier (MOPA);

FIG. 1B illustrates a schematic diagram of a target application thatuses the output of the fiber MOPA laser of FIG. 1A;

FIG. 1C illustrates an exemplary cladding pumped fiber MOPA designincluding one embodiment of a fiber delivery assembly;

FIG. 2 illustrates an exemplary fiber grating reflection spectra alongtwo polarization axes that allow polarization selection in modelockedfiber lasers having polarization maintaining fiber Bragg gratings;

FIG. 3A illustrates a schematic diagram of one embodiment of a saturableabsorber mirror;

FIG. 3B illustrates another embodiment of a saturable absorber minor;

FIG. 4 illustrates an exemplary core pumped fiber MOPA according to oneembodiment of the present teachings;

FIG. 5 illustrates another embodiment of a core pumped fiber MOPA;

FIG. 6 illustrates a block diagram of a polarization maintaining fiberMOPA design;

FIG. 7 illustrates a block diagram of a polarization maintaining fiberoscillator-amplifier design including an embodiment of a pulse deliveryassembly;

FIG. 8A illustrates an exemplary polarization maintaining fiberoscillator-amplifier coupled to a highly nonlinear fiber in conjunctionwith one embodiment of an oscillator phase control system;

FIG. 8B illustrates one embodiment of the polarization maintaining fiberoscillator of FIG. 8A wherein the oscillator design allows for phasecontrol of the oscillator;

FIGS. 8C-8E illustrate some of the possible approaches for controllingthe beat signal related to the carrier envelope offset frequenciesassociated with the system of FIG. 8A;

FIGS. 8F and 8G illustrate a schematic diagram of one embodiment of apolarization maintaining fiber oscillator that facilitates generation ofcarrier envelope offset frequency beats for precision frequency combgeneration;

FIG. 9A illustrates an exemplary measurement of a free-running carrierenvelope offset frequency obtained with a modelocked fiber laseraccording to one embodiment of the present teachings;

FIG. 9B illustrates an exemplary measurement of a phase locked carrierenvelope offset frequency obtained with a modelocked fiber laseraccording to one embodiment of the present teachings;

FIG. 10A illustrates a schematic diagram of one embodiment of a lownoise fiber oscillator design that can generate narrow bandwidth carrierenvelope offset frequency beats for precision frequency comb generation;

FIG. 10B illustrates a schematic diagram of another embodiment of a lownoise fiber oscillator design that can generate narrow bandwidth carrierenvelope offset frequency beats for precision frequency comb generation;

FIG. 11 illustrates an embodiment of a possible design of asupercontinuum device;

FIG. 12 illustrates a schematic diagram of one embodiment of a highlyintegrated frequency comb source with an in-line configuration; and

FIG. 13 illustrates a schematic diagram of one embodiment of a frequencycomb source with a single arm interferometer and a feedback systemconnected thereto.

DETAILED DESCRIPTION OF CERTAIN EMBODIMENTS

These and other aspects, advantages, and novel features of the presentteachings will become apparent upon reading the following detaileddescription and upon reference to the accompanying drawings. In thedrawings, similar elements have similar reference numerals.

Various embodiments disclosed in the present teachings relate to designand construction of modelocked fiber-based master oscillator poweramplifier (MOPA) laser systems. FIG. 1A illustrates a diagram of a fiberMOPA laser 10 comprising a fiber oscillator 16 optically coupled to apost-oscillator component 12 by a coupler 14. In various embodiments,the post-oscillator component 12 comprises a fiber amplifier. In otherembodiments, the amplifier may be omitted, and the post-oscillatorcomponent 12 may comprise one or more optical elements that conditionand deliver the output from the oscillator 16.

One aspect of the present teachings relates to a fiber Bragg gratingacting as the coupler 14 between the oscillator and the post-oscillatorcomponent 12 such as the amplifier. A bidirectional reflective opticalelement, such as a fiber Bragg grating, may counter emissions from thepost-oscillator component 12 to the oscillator 16, which may otherwisenegatively impact the performance of the oscillator. Use of the fibergrating for such a purpose may eliminate the need for a bulk componentisolator between the oscillator and the amplifier.

As shown in FIG. 1A, the fiber MOPA laser 10 yields an output signal 22in response to a pumping energy 20. In various embodiments, the output22 comprises ultrafast pulses having pulse widths in the femtosecondregime.

The present teachings also disclose some possible applications of theultrafast pulses that can be obtained from the laser system 10. FIG. 1Billustrates a diagram where the output 22 from the laser 10 is deliveredto a target 24 of interest. As described more fully below, one possibletarget comprises an interferometer that can be useful for metrologyapplications.

Various design considerations for the laser system and the targetapplications are described herein. Such design considerations include,but are not limited to, (1) well defined output polarization state; (2)construction of the fiber laser preferably adaptable to mass production;(3) use of optical elements that are preferably inexpensive; 4)techniques and designs for generation of passive modelocked signals withwell controllable parameters; as well as (5) simplified pulseamplification implementation.

FIG. 1C represents an exemplary embodiment of a fiber-based masteroscillator power amplifier (MOPA) laser system 100. The laser system 100comprises a polarization-maintaining gain fiber 101 having a core 102and a cladding region 103. The fiber core 102 can be doped withrare-earth ions such as Yb, Nd, Er, Er/Yb, Tm or Pr, to produce gain ata signal wavelength when the laser system 100 is pumped with an inputsuch as a diode laser 104. The pump diode 104 can be coupled into thecladding region 103 of fiber 101 using for example two lenses 105 and106 and a V-groove 107. The pump to fiber coupling may be achieved usingmore than two lenses, or by using any other coupling methods. The fibercore 102 can be single-mode or multi-mode.

As seen in FIG. 1C, the MOPA 100 comprises an oscillator assembly 114and an amplifier assembly 115. The oscillator assembly 114 is bounded onone end by a fiber grating 108 and on the other end by a saturableabsorber assembly 110. The amplifier assembly 115 approaches the fibergrating 108 on one end and is bounded by a fiber end 111 on the otherend. In the exemplary fiber-based MOPA laser 100, the same fiber 101 isused in both oscillator and amplifier sections 114, 115. In general,however, different fibers can be used in the oscillator 114 andamplifier 115, though to avoid feedback from the amplifier into theoscillator, the refractive index of both oscillator and amplifier fiberare preferably closely matched in various preferred embodiments.

In the exemplary laser 100, the fiber Bragg grating 108 is formed in apolarization maintaining fiber. The fiber Bragg grating 108 can bewritten directly into the fiber 101. In alternative embodiments, thefiber Bragg grating 108 can be spliced into the MOPA system 100 atsplice positions 112 and 113 between the oscillator and amplifiersections 114, 115, preferably such that the polarization axes of theinvolved fibers are aligned with respect to each other. Polarizationcross-talk between two polarization axes of the polarization maintainingfiber containing the fiber grating 108 is preferably suppressed by morethan approximately 15 dB. The fiber grating 108 can be chirped orun-chirped.

The saturable absorber assembly 110 referred to above is described morefully below. As shown in FIG. 1C, the exemplary laser 100 furthercomprises an optional polarizer 109. The use of an integrated fiberpolarizer is optional, since for example, for oscillators operating farabove threshold, single-polarization operation of the system can beobtained by the use of a polarization maintaining fiber grating (i.e., afiber grating written into polarization maintaining fiber).

This polarization maintaining concept is illustrated in FIG. 2, where anintra-cavity fiber gain as a function of wavelength inside the fiberoscillator is represented with line 200. The reflectivity along the twopolarization axes of the polarization maintaining (PM) fiber grating isrepresented with curves 201 and 202. Due to the birefringence of the PMfiber grating, the peak reflection wavelengths along the twopolarization axes differ by approximately between 1-10 nm, depending onthe birefringence of the fiber. The absolute cavity gain along the twopolarization axes is proportional to the product of the fiber gratingreflectivity and the fiber gain integrated over wavelength. Hence evenvery small wavelength dependent slopes in the fiber gain produce adifference in absolute cavity gain for the two polarization axes, andlasing will start out in the higher gain axis and suppress lasing in thelower gain axis once the gain inside the cavity is saturated.

An exemplary integrated fiber polarizer is described in a U.S. patentapplication Ser. No. 10/627,069 filed on Jul. 25, 2003, now U.S. Pat.No. 7,088,756, which is hereby incorporated herein by reference in itsentirety.

One aspect of the present teachings relates to a fiber grating (108 inFIG. 1C) being interposed between the oscillator and amplifiercomponents of a fiber-based MOPA laser. Such a fiber grating may performthree uses. First, the fiber grating may be used as an output mirror(i.e., it feeds part of the signal back to the fiber oscillator cavity114). Second, the fiber grating can control the amount of cavitydispersion as explained in the U.S. patent application Ser. No.10/627,069. Third, the fiber grating can prevent emission from theamplifier assembly 115 from perturbing output of the oscillator assembly114, thereby foregoing the need for an optical isolator separating theamplifier and oscillator.

Saturation of the gain inside a modelocked fiber oscillator 114 can beobtained when the laser is operated far above its threshold. Because ofthe small core size of optical fibers, self-phase modulation generallylimits the obtainable output power of modelocked fiber lasers; andmodelocked operation far above lasing threshold is not easilyaccomplished. In one aspect of the present teachings, the reflectivityof the PM fiber grating is reduced thereby increasing populationinversion and allowing increased output power. Modelocked laseroperation significantly above the threshold can be achieved. In oneembodiment, the PM fiber grating reflectivity is less than or equal toapproximately 60%. In various embodiments, for example, the reflectivityof the reflective optical element forming the laser cavity may be lessthan or equal to about 50%, 40%, or 30%. Smaller reflectivities arepossible. The reflectivity, may for example be less than or equal toabout 20%, 10%, 5%, or 3% in certain embodiments. Still, other ranges ofreflectivity values not specifically referred to herein are alsopossible. This relatively low reflectivity criteria is nearly theopposite to the design of single-polarization continuous-wave (CW)single-frequency lasers, where the reflectivity of the PM fiber gratingis maximized to values greater than approximately 90% to enable reliablesingle-polarization operation. In contrast, the reflectivity of thereflective optical element forming the laser cavity for embodiments ofmode-locked oscillators described herein are preferably less than 90%.

With reduced reflectivity, pump power will be increased. Generally,reliable modelocked operation in one polarization axis can be obtainedwhen the pump power supplied to the laser oscillator at modelockingthreshold exceeds the pump power for the CW laser threshold by around10-20%.

As discussed above, the fiber grating may counter emission from theamplifier assembly (115) to the oscillator assembly (114) that wouldotherwise be detrimental to the performance of the oscillator. Thereflectivity of the grating 108 can be very small and still replace anefficient isolator. Such a design criteria is in contrast toconventional single-frequency lasers. In various MOPA embodimentsdisclosed herein, a product of the reflectivities for the oscillatorcavity (one of which is the grating 108) can be as low as about 10% or5% or less, such as, for example, about 3%. Since the ratio of aphoton's cavity lifetime τ_(c) to its single-pass transit time τ_(t) canbe expressed as

τ_(c)/τ_(t)=2/[−ln(R ₁ R ₂)]  , (1)

where R₁R₂ is the product of reflectivities, R₁R₂=0.03 results in theratio τ_(c)/τ_(t)≈0.6 (e.g. with R₁≈100% and R₂≈3%). Hence, the cavitylifetime in ultrafast MOPAs can be shorter than the single-pass transittime. Such a feature is unique when compared to other MOPA designs inthe field of semiconductor lasers or CW fiber lasers.

As discussed above, low grating reflectivities enable operation of theoscillator further above threshold. Such operation above thresholdincreases the oscillator output power, reducing amplified spontaneousemission (ASE) noise, and increasing the polarization selectivity of PMfiber gratings.

The fiber grating can also be used to control the dispersion of thepulses in the fiber. As explained in the U.S. patent application Ser.No. 10/627,069, to produce the short pulses (with an optical bandwidthcomparable to or larger than the bandwidth of the gain medium), theabsolute value of the grating dispersion is preferably selected to bewithin a range of approximately 0.5-10 times the absolute value of theintra-cavity fiber dispersion, where the grating dispersion and thefiber dispersion are selected to be of opposite signs. Accordingly, theoscillation of chirped pulses is enabled in the cavity, thereby reducingor minimizing the effects of the nonlinearity of the cavity and furthermaximizing the pulse energy.

As further shown in FIG. 1C, the output of the MOPA can be coupled to adelivery fiber 118 via exemplary coupling lenses 116, 117. The chirp ofthe output pulses can be conveniently compensated with the deliveryfiber 118. The delivery fiber 118 can comprise by way of examples, astandard silica step-index fiber or a holey fiber such as a photoniccrystal fiber. The use of a photonic crystal fiber for dispersioncompensation and pulse delivery is disclosed in U.S. patent applicationSer. No. 10/608,233 filed on Jun. 30, 2003, now U.S. Pat. No. 7,257,302,which is hereby incorporated herein by reference in its entirety. Thedelivery fiber 118 can also be spliced directly to the fiber end face111 of the amplifier assembly 115, thereby enabling a furtherintegration of the laser assembly. Compressed pulses can thus be outputfrom the delivery fiber.

FIGS. 3A and B now illustrate two possible embodiments of the saturableabsorber assembly 110 referred to above in reference to FIG. 1C. In oneembodiment as shown in FIG. 3A, a saturable absorber module (SAM) 300includes an InGaAsP layer 301 with a thickness of approximately 50-2000nm. The layer 301 can be grown with a bandedge in the 1 μm wavelengthregion; the wavelength is defined by the sought emission wavelength ofthe fiber laser and can vary between approximately 1.0-1.6 μm. TheInGaAsP layer 301 can be further coated or processed with a reflectivematerial such as Au or Ag. Preferably, a dielectric mirror or asemiconductor Bragg reflector 302 is located adjacent the layer 301, andthe entire structure is mounted to a heat sink 303 based on, forexample, metal, diamond or sapphire.

The InGaAsP layer 301 can further be anti-reflection coated with a layer304 on its surface opposite from the reflecting surface 302 to optimizethe performance of the SAM 300. Because of the saturable absorption bythe InGaAsP layer 301, the reflectivity of the SAM 300 increases as afunction of light intensity, which in turn favors the growth of shortpulses inside the laser cavity. The absence of Al in the saturableabsorber layer 301 prevents oxidization of the semiconductor surfaces inambient air, and thus enhances the lifetime and power handlingcapability of the structure.

Instead of InGaAsP, any other saturable semiconductor material can beused in the construction of the SAM. Also, Al-containing semiconductorscan be used in the SAM with appropriately passivated surface areas.Surface passivation can, for example, be accomplished by sulfidizationof the semiconductor surface, encapsulating it with an appropriatedielectric or with an Al-free semiconductor cap layer. An AlGaInAsabsorber layer grown lattice-matched on InP can be surface-passivatedwith a thin (about 10 nm range) cap layer of InP. AlGaInAs with a higherbandgap energy than the absorber layer can also be used for asemiconductor Bragg reflector in combination with InP. Among conceptsfor semiconductor Bragg mirrors lattice-matched to InP, an AlGaInAs/InPcombination has an advantage over an InGaAsP/InP Bragg reflector due toits high refractive index contrast.

Instead of a bulk semiconductor saturable absorber, a multi-quantum well(MQW) saturable absorber structure 305 as shown in FIG. 3B may also beused. The SAM 305 conveniently comprises MQW structures 306, 307 and 308interleaved with passive spacer layers 309, 310, 311, and 312 in orderto increase the saturation fluence and depth-selective ion-implantationconcentration of each MQW section. Additional MQW structures can beused, similarly separated by additional passive spacer layers. To reducethe wavelength and location sensitivity of the MQW saturable absorbers,the width of the spacer layers can vary from spacer layer to spacerlayer. Furthermore, multiple bulk layers with thicknesses larger thanapproximately 500 Å can replace the MQW structure. The MQW layers, inturn, can comprise several layers of quantum wells and barriers such as,for example, InGaAs and GaAs, or InGaAsP/InP and AlGaInAs/InP,respectively.

The exposed surface of the layer 309 can further be anti-reflectioncoated (not shown). The reflective function of the structure 305 can beobtained by including a minor structure 313. The entire structure can bemounted on a heat sink 314.

The control of the response time of the saturable absorption forconcomitant existence of fast and slow time constants can be realized byintroducing carrier trap centers with depth controlled H+ (or other ion)implantation. The implantation energy and dose can be adjusted such thatpart of the absorbing semiconductor film contains a minimal number oftrap centers. For example, the semiconductor layer with the minimalnumber of trap centers can be selected to be at the edge of the opticalpenetration range of exciting laser radiation. Such a design serves onlyas an example, and alternatively any semiconductor area within theoptical penetration range can be selected to contain a minimal number oftrap centers. Hence, distinctive bi- or multi-temporal carrierrelaxation can be obtained in the presence of optical excitation.

Additional details about the structures and properties of the saturableabsorber assemblies are described in the U.S. patent application Ser.No. 10/627,069 which is incorporated herein by reference in itsentirety. Other saturable absorber designs, however, not specificallyrecited herein, such as those known in prior art or yet to be devised,are also possible.

Some possible design examples of the fiber MOPA laser are now described.In one exemplary design (Design Example #1) corresponding to the fiberMOPA laser system of FIG. 1C, the polarization-maintaining fiber 101 isdoped with Yb with a doping level of approximately 1% by weight. Thediameter of the fiber core 102 is approximately 6 μm, and the cladding(103) diameter is approximately 125 μm. The intra-cavity length of thedoped fiber is approximately 1.0 m. An additional approximately 1.0 mlength of undoped polarization-maintaining fiber is incorporatedintra-cavity in place of the fiber polarizer 109. The overall (summed)dispersion of the two intra-cavity fibers is approximately +0.09 ps².

In contrast, the single-mode polarization maintaining fiber grating 108has a dispersion of approximately −0.11 ps² to approximately match thefiber dispersion, a spectral bandwidth of approximately 25 nm, and areflectivity of approximately 10% centered at approximately 1050 nm. Thegrating 108 was manufactured with a phase mask with a chirp rate ofapproximately 350 nm/cm. The fiber grating 108 is spliced into the MOPAusing the splices 112 and 113. The length of fiber from the position ofthe grating 108 to the splices is less than approximately 5 cm.

The total oscillator dispersion is approximately −0.02 ps² (fiberdispersion+grating dispersion). The extra-cavity length of doped fiber101 is approximately 30 cm. The MOPA is pumped through the V-groove 107with a pump power up to approximately 1 W at a wavelength ofapproximately 976 nm.

The saturable absorber element 110 comprises a film of InGaAsP grown onan InP substrate. The bandedge of the InGaAsP film is at approximately1050 nm. The InGaAsP film thickness is approximately 500 nm, and theshort lifetime of the absorber is approximately 1 ps and the second longabsorber lifetime is around 150 ps.

The laser produces chirped optical pulses with a full-width half maximumwidth of approximately 1.5 ps at a repetition rate of approximately 50MHz with an average power of approximately 60 mW. The pulse spectralbandwidth is around 25 nm and thus the pulses are around 10 times longerthan the bandwidth limit, which corresponds to around 100 fs. Thegeneration of pulses with a pulse width of approximately 140 fs isenabled by the insertion of the pulse compressor/delivery fiber 118 atthe output of the MOPA. Details on such pulse compressing fibers aredisclosed in the U.S. patent application Ser. No. 10/608,233.

The output of the MOPA is coupled into the compressor/delivery fiber 118using the lenses 116 and 117. In this design example, a fiber with acentral air-hole is used to reduce self-phase modulation in thecompressor fiber 118. Alternative pulse compressor elements, such asfiber gratings, bulk gratings, bulk volume gratings, chirped minors orprism pairs can be used instead of the compressor fiber 118 to providepulse compression. Still other designs are possible. Furthermore,although in this design example a side-pumped MOPA system is disclosed,other pumping methods such as end-pumping and/or pumping through fibercouplers can be used.

FIG. 4 illustrates another embodiment of a particularly stablesingle-mode pumped fiber MOPA laser 400. The MOPA 400 comprisessubstantially identical composition Er oscillator and amplifier fibers401 and 402. The MOPA is pumped via a pump laser 403 through a pigtail404 and a polarization-maintaining wavelength division multiplexing(WDM) coupler 406.

An alternative method for the isolation of the signal light from thepump port is to use a thin film-based WDM filter as the coupler 406.Such a filter has dielectric coatings on both sides where the coating onone side forms a low or high band pass filter. In one embodiment of sucha device, the filter provides high reflectivity for the signalwavelength, while high transmission is provided for the lower pumpwavelength. However, any leakage of the signal light into the pump portis generally not suppressed by more than approximately −25 dB in such abasic thin-film WDM. A preferred embodiment of a thin film WDM couplercomprises a multiple stack of thin film WDM filters for the coupler 406for higher signal isolation. For example, with the addition of twoadditional filters to the WDM, the signal isolation into the pump portcan be increased by approximately −70 dB. Thin film WDMs are known inthe art, and the stacking of several thin film filters in such a WDM isa straight-forward extension of the design of such standard WDMs, andtherefore is not separately shown. Standard WDM couplers based onevanescent coupling can also be utilized.

The pump light is passed through the amplifier fiber 402 and through apolarization-maintaining (PM) grating 407 to the oscillator fiber 401.The oscillator assembly also comprises an undoped fiber section 408interposed between the oscillator fiber 401 and a saturable absorbermodule 409.

The output from the MOPA is directed via the WDM 406, through apolarization-maintaining (PM) pig-tail 410 and a PM isolator 411,towards an output end of the system. An optional frequency conversionelement 412 comprising for example a periodically poled LiNbO₃ (PPLN)can be inserted at the output end of the system, where lenses 413, 414,and 415 are used to obtain an appropriate beam diameter inside the PPLNcrystal 412. All fiber elements of the MOPA laser 400 are connected withPM splices denoted as 416, 417, 418, 419, and 420.

Although Er-doped fibers are used in the MOPA laser 400 of FIG. 4,similar single-mode pumped systems can be constructed using otherrare-earth gain media such as Yb, Nd, Tm, Pr, Ho and combinationsthereof. Similarly, whereas a diode pump laser (403) is used forpumping, other solid-state or fiber based pump lasers may also be used.Still other configurations are possible.

In one exemplary design (Design Example #2) corresponding to the fiberMOPA laser system of FIG. 4, the length of the oscillator and amplifierfibers 401, 402 are approximately 1 meter and 0.40 meter, respectively.The thin-film based WDM filter provides high reflectivity for the signalwavelength around 1550 nm, while high transmission is provided for thepump wavelength around 980 nm. It should be understood, however, thatthe application of such a pump-signal combiner WDM is not limited to1550 nm and 980 nm for the signal and pump lines. The PM grating 407 ischirped with a reflectivity of approximately 10% and a dispersion ofapproximately −0.12 ps². The undoped fiber section 408 has a length ofapproximately 1 m. The saturable absorber module 409 comprises a bulkInGaAsP saturable absorber mirror. The InGaAsP film has a bandedge ofapproximately 1560 nm, and the two saturable absorber lifetimes areoptimized with depth selective ion implantation as approximately 1 psand 130 ps. The foregoing operating parameters result in the MOPA system400 producing an output power up to approximately 100 mW at a wavelengthof approximately 1560 nm, amplifying the oscillator power by more than afactor of three.

FIG. 5 illustrates a variation to the MOPA design 400 described above inreference to FIG. 4. The MOPA design 400 shown in FIG. 4 comprises apump source delivered first to an amplifier and then to an oscillator.Alternatively, as shown in FIG. 5, a MOPA design 500 comprises a pumpsource 503 delivered first to an oscillator 501 and then to an amplifier502. The pump laser 503 is delivered to the oscillator through a PM WDM504. A PM fiber grating 505 is interposed between the oscillator 501 andthe amplifier 502. A saturable absorber module 506 is disposed on theend of the oscillator 501 opposite from the fiber grating 505.

The output from the MOPA is extracted through an isolator 507. A designas shown in FIG. 5 may be used to insert an optical isolator between theoscillator 501 and the amplifier 502 without the need for an additionalpump source. In such a design, the isolator can be disposed between theamplifier 502 and the fiber grating 505. For example, when using anEr-fiber MOPA system, pumping at a wavelength of approximately 1480 nmis possible, while the emission wavelength of Er is approximately in the1.55 μm wavelength region. Due to the small difference in wavelengthbetween pump and emission, such an isolator can isolate at bothwavelengths without the introduction of large optical losses.

It will be appreciated that many alternative embodiments forcladding-pumped or core pumped fiber MOPAs are possible. FIG. 6 nowillustrates a generalized MOPA design of the present teachings. Ageneric MOPA 600 comprises a modelocked fiber oscillator 601 that uses afiber grating 602 as an output coupler. No isolator is used between theoscillator 601 and an amplifier 603 that is inline with the fibergrating 602 and the oscillator 601.

The fiber amplifier 603 further amplifies the pulses from oscillator601. An optional isolator 604 suppresses feedback from any spuriousreflections outside the system.

Both oscillator and amplifier are preferably constructed frompredominantly polarization maintaining fiber. That is, any length ofnon-polarization maintaining fiber is preferably kept smaller thanapproximately 10 cm to prevent de-polarization in such non-polarizationmaintaining fiber sections. A single pump source can be used to pumpboth oscillator and amplifier, although separate pump sources coupled tothe oscillator and the amplifier can also be implemented.

FIGS. 7-12 illustrate various applications of the output signals thatcan be generated by the various embodiments of the fiber MOPA lasersdescribed above in reference to FIGS. 1-6. It will be understood thatsuch applications are in no way intended to limit the applicability ofthe laser designs of the present teachings.

For many applications of ultrafast fiber lasers, a delivery ofultrashort pulses in a single polarization state is desired. FIG. 7illustrates a generic system 700 that can deliver such pulses. Thesystem 700 comprises a modelocked fiber oscillator 701 and a fiberamplifier 702, where the oscillator 701 produces a well-definedpolarization state, which is preferably linear. The amplifier 702comprises a polarization-maintaining fiber, and preferably produceschirped linearly polarized pulses.

The system 700 further comprises lenses 703 and 704 that couple theoutput from the fiber amplifier 702 to a delivery fiber 705 that ispreferably also polarization maintaining. The dispersion of the deliveryfiber 705 can be used to produce optimally short pulses at an opticaltarget 706 downstream from the oscillator amplifier system. An opticalsystem 707 can be used to image the output of the delivery fiber 705onto the target 706. To enable increased or maximum power handling ofthe fiber delivery system, the delivery fiber 705 is preferablyconstructed from a fiber with a guiding air hole such as a hollow corephotonic band gap fiber.

MOPA's such as described herein can be used to create an accurate andstable reference source of standard wavelength and frequencies. Suchstandard wavelengths and frequencies can be employed, for example, inresearch related activities such as metrology as well as otherapplications. For such applications in metrology, the generation ofcoherent optical spectra with a spectral width of one octave and more isof particular interest. The use of a modelocked laser allows thegeneration of combs of well-defined frequencies. The generated frequencycomb is considered to be well-defined if two degrees of freedom, namelythe repetition frequency of the laser f_(rep) and the carrier envelopeoffset frequency f_(ceo), can be measured or preferably stabilized to anexternal clock. See, for example, Steven T. Cundiff et al, “OpticalFrequency Synthesis Based on Mode-locked Lasers”, Review of ScientificInstruments, vol. 72, no. 10, October 2001, pp. 3749-3771 as well asSteven T. Cundiff et al, “Femtosecond Optical Frequency Combs,” Reviewsof Modern Physics, vol. 75, January 2003, pp. 325-342, which are herebyincorporated herein by reference in their entirety. The frequency of onecomb line can be expressed as

f _(n) =n f _(rep) +f _(ceo)  (2)

where n is the integer identifying the single comb line. Measurement andstabilization of f_(ceo) is possible, for example, with an octavespanning external broadened modelocked laser source and an f to 2finterferometer. Schemes for the self-referencing stabilization off_(ceo) are described for example in H. R Telle et al. Appl. Phys. B 69,327-332 (1999), which is hereby incorporated herein by reference in itsentirety.

Frequency combs spanning one octave can be generated by supercontinuumgeneration in nonlinear fibers. For industrial applications of frequencycombs, the generation of supercontinua from modelocked fiber or generalwaveguide lasers may be particularly advantageous. As described above,various polarization maintaining and highly integrated fiber laserdesigns of the present teachings allow the generation of coherentcontinua.

FIGS. 8A and 8B illustrate an exemplary frequency comb source 800 thatgenerates such a polarization maintaining continuum (or frequency comb).As shown in FIG. 8A, the frequency comb source 800 comprises apolarization maintaining fiber oscillator 801 which is described belowin greater detail in reference to FIG. 8B. An output from the oscillator801 is directed via an isolator 802 to a polarization maintaining fiberamplifier 803. With carefully designed systems, such as some of thefiber lasers described above, the isolator 802 may be omitted andsimilar protection of the oscillator from spontaneous emissions from theamplifier may be achieved by a fiber Bragg grating which may evenpossess only small reflectivity (R≈3%) in some cases.

For applications in metrology, the oscillator 801 and the amplifier 803are preferably pumped by two different pump sources, allowingindependent control thereof although other configurations are possible.The pump source (not shown) for the fiber amplifier 803 is preferablysingle-mode or has a very low residual noise. The pump source isinjected into the amplifier 803 via a polarization maintainingwavelength division multiplexing coupler (not shown). Alternatively, acladding-pumped fiber amplifier can also be implemented. Otherarrangements can be employed as well.

As shown in FIG. 8A, the fiber amplifier 803 is connected via a splice804 to a highly nonlinear fiber (HNLF) 805. The highly nonlinear fiber805 is preferably constructed from a holey fiber or a standard silicafiber or using bismuth-oxide based optical glass fiber in variousembodiments. The dispersion of the highly nonlinear fiber 805 ispreferably close to approximately zero at the emission wavelength of theoscillator 803 for certain designs. Even more preferably, the dispersionprofile is flattened, i.e., the third-order dispersion of the fiber 805is equally close to approximately zero. The highly nonlinear fiber 805does not need to be polarization maintaining since it is relativelyshort (on the order of few cm long), thereby enabling long-termpolarization stable operation. The length of the highly nonlinear fiber805 is preferably selected to be less than approximately 20 cm topreserve the coherence of the generated continuum. Other designs,however, are possible.

The (continuum) output from the highly nonlinear fiber 805 is injectedvia a splice 806 to a wavelength division multiplexing coupler 807. Thecoupler 807 directs the long and short wavelength components from thecontinuum to a long wavelength coupler arm 808 and a short wavelengthcoupler arm 809 respectively. The long wavelength components aresubsequently frequency doubled using exemplary lenses 810, 811, 812,813, as well as a doubling crystal 814. After frequency doubling theresulting output preferably has a substantially same wavelength as atleast part of the short wavelength components traveling in the arm 809.Additional optical elements 815 and 816 can be inserted into the beampaths of the arms 808 and 809 for spectral filtering, optical delayadjustment, as well as polarization control. Spectral filtering elementsare selected to maximize the spectral overlap of the signals propagatingin arms 808 and 809. As another example, the optical element 815 cancomprise appropriate wave-plates that control the polarization state ofthe light in front of the doubling crystal 814.

The frequency-doubled light from the arm 808 and the light from the arm809 are subsequently combined in a polarization-maintaining coupler 817which preferably has a 50/50 splitting ratio. The beat signal frominterference of the two beams injected into the coupler 817 is detectedby a detector 818. The beat signal is related to the carrier envelopeoffset frequency f_(ceo). The radio frequency (RF) spectrum of thedetector signal comprises spectral components around f_(n,rep)=n f_(rep)which are related to the repetition frequency of the laser, and aroundf_(n,m,beat)=n f_(rep)+m f_(ceo) which are related to the beat signalbetween the repetition frequency f_(rep) and the carrier envelope offsetfrequency f_(ceo). In both cases the integer n indicates the harmonicnumber, and m has the value of −1 or +1.

As shown in FIG. 8A, one selected harmonic of the beat signal atfrequency f_(n,m,beat) may be directed via an electrical feedbackcircuitry 819 to the oscillator 801 and provides the necessary feedbackto allow control of the carrier envelope offset phase. One selectedharmonic of the repetition frequency signal at f_(n,rep) may be used ina similar way to control the repetition rate of the oscillator 801.

To minimize the noise of the beat signal and to maximize the amplitudeof the beat signal, additional elements may be used. For example, toincrease or optimize the power of the frequency-doubled light, thedoubling crystal 814 is preferably based on a periodically poledmaterial such as a periodically poled LiNbO₃ (PPLN) in variousembodiments. The length of the crystal 814 is preferably selected in arange of approximately 3-75 mm to minimize the spectral bandwidth of thefrequency-doubled signal. Three-wave mixing in the PPLN greatlyincreases the acceptance bandwidth for frequency doubling, whileensuring a narrow bandwidth output. Such an effect is disclosed in theU.S. Pat. No. 5,880,877 to Fermann et al which is hereby incorporatedherein by reference in its entirety. The generation of a narrowbandwidth output signal reduces the number of frequency componentsinvolved in the generation of a carrier envelope offset frequency beatand enhances the signal-to-noise ratio of the beat signal.

The beat signal can be further increased or maximized by ensuringtemporal overlap between the interfering pulses impinging on thedetector 818. Increased temporal overlap can be achieved by anadjustment of the length of coupler arm 809. Alternatively, additionalglass (or semiconductor) plates can be inserted (for example, at theoptical element 815) in the optical beam paths to obtain similarresults.

An optical element 816 a may be inserted in an optical path after thetwo arms 808, 809 are combined. The optical elements 816 and 816 a thatcan be inserted into the arm 809 and in the combined signal arm beforethe detector 818 may comprise a narrow bandpass filter that narrows thespectral width of the signal transmitted through the arm 809. Suchfiltering can reduce or minimize the noise of the beat signal as well asavoid saturation of the detector 818. Convenient narrow bandpass filterscan be constructed from, by way of examples, fiber gratings, bulkgratings or dielectric filters.

The radio frequency (RF) spectral width of the beat signal related tothe carrier envelope offset phase slip can be reduced or minimized in anumber of ways. One way is by the incorporation of a fiber grating intothe fiber oscillator as well as a bandwidth-limiting element.

Another method for reducing the RF spectral width of the beat signal isto reduce the contribution of Raman processes in the continuum output.This task can be accomplished, for example, by using for continuumgeneration a short piece of nonlinear fiber comprising highly nonlinearBismuth-oxide based optical glass with non-zero dispersion, preferablypositive dispersion, at the emission wavelength of the laser.

Another way to reduce or minimize the RF spectral width of the beatsignal is by reducing the dispersion of the seed oscillator. In thepresence of a low oscillator dispersion, phase noise and consequentlytiming jitter of the oscillator are reduced or minimized, which in turnreduces or minimizes the variation of the carrier envelope beatfrequency. For such fiber oscillators, the oscillator dispersionpreferably is in a range of approximately (−10,000 to +2,500 fs²/m)×L,where L is the intra-cavity fiber length. As an example, a Fabry-Perotoscillator operating at a repetition rate of 50 MHz and having anintra-cavity fiber length of L=2 m, the oscillator dispersion preferablyshould be in a range from approximately −20,000 fs² to +5,000 fs².

To produce an optical output of the frequency comb source which is used,for example, for a frequency metrology experiment, part of the frequencycomb can be coupled out from a location 818 b after the highly nonlinearfiber or from a location 818 a after the coupler 817 and interferometer.The output can be coupled out, e.g., with broad band fiber opticcouplers or with WDM fiber optical couplers, if for example only acertain spectral part of the comb is used. Other types of outputcouplers such as bulk optics or fiber Bragg gratings can also be used.The optical output can also be coupled out at a location 818 d after theoscillator or at a location 818 c after the amplifier, if for exampleonly the spectral part of the oscillator or amplifier bandwidth of thecomb is desired.

The amount and wavelength range of the optical signal out coupled ispreferably chosen such that the beat signals obtained from detector 818have sufficiently high signal/noise ratio for providing an effectivefeedback loop 819.

FIG. 8B now illustrates one possible embodiment of the oscillator 801described above in reference to FIG. 8A. Such a design facilitates thegeneration of well-defined frequency combs. The design is very similarto the design described above in reference to FIGS. 1C and 2, and theuse of a fiber grating allows a particularly compact construction.

The oscillator 801 includes a saturable absorber module 820 comprisingcollimation and focusing lenses 821 and 822 respectively. The saturableabsorber module 820 further comprises a saturable absorber 823 that ispreferably mounted onto a first piezo-electric transducer 824. The firstpiezo-electric transducer 824 can be modulated to control, for example,the repetition rate of the oscillator 801.

The oscillator 801 further comprises an oscillator fiber 825 that ispreferably coiled onto a second piezo-electric transducer 826. Thesecond piezo-electric transducer 826 can be modulated for repetitionrate control of the oscillator 801. The oscillator fiber 825 ispreferably polarization-maintaining and has a positive dispersionalthough the designs should not be so limited. The dispersion of theoscillator cavity can be compensated by a fiber grating 827 whichpreferably has a negative dispersion and is also used for outputcoupling. It will be understood that a positive dispersion fiber gratingand a negative dispersion cavity fiber may also be implemented.Furthermore, the fiber grating 827 can be polarization-maintaining ornon-polarization-maintaining.

A linear polarization output from the oscillator 801 can be obtainedusing appropriate splicing techniques as discussed above in reference toFIG. 1C and also as discussed in the U.S. patent application Ser. No.10/627,069.

A fiber grating is preferred as a dispersion compensation elementcompared to a piece of fiber, in various embodiments, because a fibergrating enables essentially linear dispersion compensation as well asbandwidth limitation. Linear dispersion is provided in an opticalelement that does not produce substantially any nonlinear distortions.An optical fiber, for example, also produces dispersion; however, afiber is much longer and therefore the pulses propagating in the fiberare subject to nonlinear distortions. In contrast, a fiber grating isrelatively short and any nonlinear distortions may be neglected (atleast for the power levels obtainable with a modelocked oscillator). Areduction of the bandwidth of the laser reduces the amount of timingjitter inside the laser, and the linear dispersion compensation allows amore stable laser operation with increased pulse energy inside thecavity.

Instead of a fiber grating, a bulk grating dispersion compensationelement (not shown) could also be incorporated into the cavity. Forexample, a bulk grating dispersion compensation element comprising twoparallel bulk gratings could be incorporated between the lenses 821 and823 in the saturable absorber module 820. Furthermore, as an alternativeto the bandwidth-limiting by the fiber grating, an optical bandpassfilter (not shown) could also be incorporated inside the cavity. Forexample, such a filter could also be located between the lenses 821 and823. Other designs and the use of other dispersive components are alsopossible.

The pump light for the oscillator 801 can be directed via apolarization-maintaining wavelength division multiplexing coupler 828from a coupler arm 829 attached to a preferably single-mode pump diode830. The pump current to the pump diode 830 can be modulated tostabilize the beat signal frequency and the carrier envelope offsetfrequency using feedback based on the signal at one selected frequencyf_(n,m,beat) or f(n,m, beat) from the detector 818. This selectedfrequency f(n,m,beat) can be isolated from the detector signal using anRF bandpass filter which rejects other harmonics of f_(n,m,beat) orf_(n,rep).

FIGS. 8C-D illustrate some of approaches to using the beat signalfrequency to control the repetition rate as well as the carrier envelopeoffset frequency. As shown in FIG. 8C, a pump current 840 can bechanged, wherein a change in the pump current can cause a change of thebeat signal frequency and more particularly the carrier envelope offsetfrequency. This dependence can be used to phase lock the beat signalfrequency (e.g., n f_(rep)+m f_(ceo)) to an external clock in a similarway as a voltage-controlled oscillator in a traditional phase lockedloop can be used. This external clock may comprise for example a Rbclock which provides a 10 MHz reference phase locked to a Rb microwavetransmission. The clock and reference can be coupled to a frequencysynthesizer that provides the suitable frequency for a phase detectorthat locks the frequency from the synthesizer with the beat signalfrequency. In various preferred embodiments, the beat signal frequencymay be multiplied, divided or otherwise processed appropriately tofacilitate locking to a reference. The clock reference for phase lockingf_(ceo) can also be a harmonic or sub-harmonic of the repetition rate ofthe oscillator f_(rep). Using this scheme, a frequency comb line can belocked to a stable optical reference and the repetition rate of theoscillator can be used as an RF signal which is then frequency locked tothe optical reference (optical clock).

As shown in FIGS. 8D and 8E, the absolute position of the f_(ceo) can becontrolled by adjusting the temperature of the fiber grating 827 with aheating element 842. Alternatively, pressure applied to the fibergrating 827 can also be used to set the f_(ceo) using for example apiezo-electric transducer 844. Since the pressure applied to the fibergrating 827 can be modulated very rapidly, modulating the pressure onthe grating 827 can also be used for phase locking f_(ceo) to anexternal clock. Slower drifts of f_(ceo) can then be controlled byadditionally varying the temperature of the fiber grating 827,preferably ensuring that the modulation of the laser diode current stayssubstantially always within the modelocking range of the oscillator 801.Preferably, such a grating 827 used to introduce controllable amounts ofphase shift and thereby control f_(ceo) by applying pressure ortemperature changes thereto is chirped.

As shown in FIG. 8B, the dependence of the repetition rate of the laseron the voltage at the piezo-electric transducers 826 and 824 can be usedto phase lock one selected harmonic f_(n,rep) of the photodetectorsignal to an external clock. Such transducers preferably alter thecavity length and thereby adjust the repetition rate f_(n,rep). Thetransducers, may, however, affect the f_(ceo) as well. For example,applying pressure to the fiber may cause stress that varies the index ofrefraction and dispersion of the optical fiber. Resultant changes inphase may shift f_(ceo). With several feedback loops for f_(n,rep) andf_(ceo), however, the dependency of both f_(n,rep) and f_(ceo) on thestimulus provided by the transducers can be decoupled. The decouplingcan be achieved by operating the feedback loops at different bandwidthor using orthogonalization circuitry. Other techniques for decouplingmay be employed as well.

Although several configurations for controlling the f_(ceo) and f_(rep)based on feedback from the interferometer are described above, otherapproaches are possible. For example, instead of varying pump power tothe oscillator to varying self-phase modulation and thereby influencethe phase to adjust f_(ceo), other mechanisms that influence phase maybe employed as well. Other components can be disposed in the path of thesignal to introduce controllable phase variation. Other techniques forvarying f_(rep) may be employed as well. Multiple feedback loops havingdifferent response times may also be utilized to provide effectivecontrol.

FIGS. 8A and 8B describe the basic design of a frequency comb sourcebased on a low noise phase-locked fiber laser for frequency metrology.Several modifications to this basic design can be easily implemented asdescribed below.

FIG. 8F illustrates one embodiment of a fiber based continuum source 850where the amplifier (803 in FIG. 8A) is omitted. Since continuumgeneration may involve a pulse energy of around 1 nJ, the amplifier 803can be omitted in such a fiber based continuum source. Absolute phaselocking with an oscillator-only system may be more stable compared to anoscillator-amplifier system and therefore preferable in certainapplications. In the exemplary continuum source 850, high qualitysub-200 fs pulses are preferably injected into a highly nonlinear fiber854 (805 in FIG. 8A). To generate such short pulses, the oscillator-onlycontinuum source 850 preferably generates positively chirped pulses inthe oscillator 801, which are compressed in an appropriate length of anegative dispersion fiber 852 before injection into the highly nonlinearfiber 854. For the oscillator-only continuum source 850, the amplifieris thus substituted with the negative dispersion fiber 852. Suchnegative dispersion fiber can be constructed from, by way of example,conventional silica fiber, holey fiber or photonic bandgap fiber. Eventhe oscillator-amplifier continuum source 800 of FIG. 8A can benefitfrom the insertion of a length of dispersion compensation fiber (notshown in FIG. 8A) before injection into the highly nonlinear fiber 805.

As shown in FIG. 8F, the oscillator-only continuum source 850 furthercomprises an interferometer 856 that interferes the two frequencycomponents as described above. The interferometer 856 may be similar tothe two-arm interferometer shown in FIG. 8A (fiber based or equivalentbulk optics components), or may be similar to a one-arm interferometerdescribed below. The output of the interferometer 856 can be detected bya detector 858, and selected signals from the detector 858 can be usedfor feedback control 860 in a manner similar to that described above inreference to FIGS. 8A-8E. FIG. 8F additionally shows possible comboutputs 860 a, 860 b, and 860 c.

FIG. 8G illustrates an exemplary one-arm interferometer 870. Such aninterferometer can be obtained by removing one of the arms (arm 809 inFIG. 8A) and modifying the remaining arm. As shown in FIG. 8G, theinterferometer 870 comprises a group delay compensator 872 inline with adoubling crystal 874. The group delay compensator 872 receives acontinuum signal from a highly nonlinear fiber located upstream, andensures that the frequency doubled and non-doubled spectral componentsfrom the continuum that are output from doubling crystal overlap intime. Moreover, since the doubled and non-doubled spectral componentsare selected to overlap in optical frequency, these components interfereand the interference signal is detected with a detector downstream.

In the exemplary one-arm interferometer setup of FIG. 8G, thetime-overlap of the two interfering signals can be controlled by thegroup delay compensator 872 that compensates the group delay of thedoubled and non-doubled frequency components acquired during thepropagation in the highly nonlinear fiber and in the doubling crystal.The group delay compensator 872 provides an appropriate negative groupdelay. If for example the dispersion is such that the group velocity ofthe frequency doubled components exceed the group velocity of thenon-doubled frequency components, the group delay compensator preferablyincludes a medium having opposite dispersion such that the groupvelocity of the frequency doubled components is lower than that of thenon-doubled frequency components so to provide suitable compensation.Preferably, suitable phase delay is provided such that the frequencydoubled pulses and the non-frequency doubled pulses substantiallyoverlap in time to obtain increased or maximal interference. This groupdelay compensator may be implemented by one or more components, whichmay include, for example, dispersive optical fiber, waveguides, or othertransmissive or reflective optical elements. When using dispersiveoptical fibers or waveguides, those elements can be spliced directly tothe highly nonlinear fiber, ensuring a compact set-up.

In the exemplary one-arm interferometer setup of FIG. 8G, the deliveryfiber to the photodetector can be omitted. However, fiber delivery ispreferred to facilitate spatial overlap of the two interfering signals.

In one exemplary design (Design Example #3) corresponding to thesupercontinuum generation and absolute phase locking according to FIG.8A, the oscillator delivers approximately 2 ps positively chirped pulseswith a bandwidth of approximately 20 nm at a repetition rate ofapproximately 51 MHz and a wavelength of approximately 1.56 μm. Theoscillator fiber 825 comprises an approximately 1 m long Er-doped fiberwith a dispersion per unit length of approximately +70,000 fs²/m. Thefiber grating 827 has a dispersion of approximately −100,000 fs² and areflection bandwidth of approximately 40 nm centered at a wavelength ofapproximately 1560. The reflectivity of the grating is approximately10%. An additional approximately 1 m long fiber-grating pigtail having adispersion per unit length of approximately −20,000 fs²/m is part of theoscillator cavity 801. The overall cavity dispersion is thus ≈0 fs² andsubstantially smaller than the absolute dispersion of the various cavityelements.

The working example (Design Example #3) uses anon-polarization-maintaining oscillator fiber as well asnon-polarization-maintaining fiber couplers. Therefore an additionalwaveplate and a rotatable polarizer are inserted between the lenses 821and 822 (polarizer adjacent the lens 822, and waveplate adjacent thelens 821). The addition of polarization control further allowsexploitation of nonlinear polarization evolution inside the laser tostabilize modelocked operation. Additional information about thenonlinear polarization evolution in fiber lasers can be found in theU.S. Pat. No. 5,689,519 to Fermann et al which is hereby incorporatedherein by reference in its entirety.

The saturable absorber 823 is substantially similar to that describedabove in reference to the working Design Example #2. Here, because ofthe short lifetime of the saturable absorber 823, nonlinear polarizationevolution can also be used as a nonlinear limiting mechanism, furtherreducing the noise of the oscillator 801 and the noise of the generatedcarrier envelope offset frequency. The oscillator 801 may be pumped witha power up to approximately 500 mW.

The amplifier 803 has a length of approximately 1 m, a dispersion ofapproximately −20,000 fs², and a core diameter of approximately 9 μm tominimize the nonlinearity of the amplifier 803. The amplifier 803 inthis working example is polarization-maintaining. The amplifier 803 ispumped with a power up to approximately 400 mW.

After the amplifier 803, approximately sub-70 fs pulses with an averagepower up to approximately 120 mW are obtained. The amplified pulses areinjected into the highly nonlinear fiber 805. The length of fiber 805 isapproximately 10 cm long, and the fiber 805 has a second-orderdispersion per unit length less than approximately 2000 fs²/m, where arange of −5000 fs²/m to +2000 fs²/m is preferable. The highly nonlinearfiber 805 yields a coherent continuum spanning from approximately1100-2300 nm.

For frequency doubling the approximately 2200 nm wavelength light, anapproximately 4 mm length of PPLN crystal 814 is used. The non-doubledlight at approximately 1100 nm from the interferometer arm 809 and thedoubled light from the arm 808 are combined and overlapped temporally inthe coupler 817. The resulting carrier envelope offset frequency beatsignal has a spectral width of approximately 200 kHz, which allows forabsolute phase-locking of the oscillator 801 via modulation of the pumpcurrent to the oscillator 801 with a phase-locked loop with a bandwidthof approximately 300 kHz.

FIG. 9A illustrates a typical measurement of a carrier envelope offsetfrequency beat signal. Here both the frequency related to the repetitionrate of the laser at approximately 51 MHz is shown as well as the beatsignals at approximately 47 MHz and 55 MHz. The spectral width of thebeat signal less than approximately 200 kHz is comparable to the widthsachieved with frequency comb sources based on conventional Ti:sapphirelasers. Hence this result further illustrates the unique utility offiber based supercontinuum sources. The significant advantage obtainedby packaging a frequency comb source based on fiber lasers does notcorrespond to a reduction in precision. The generation of a narrowbandwidth carrier envelope offset frequency beat signal is important forthe long-term stability and accuracy of the frequency combs generatedwith a modelocked fiber laser incorporating absolute phase control.

FIG. 9B illustrates an exemplary RF spectrum of a frequency comb sourcedescribed above in reference to FIG. 8A, with a locked carrier envelopeoffset frequency. Here the carrier envelope offset frequency atapproximately 60 MHz is locked to a frequency standard at approximately10 MHz which is locked to the Rb-atomic transition. Another frequencysynthesizer is locked to this frequency standard to provide thereference signal at approximately 60 MHz. Various RF resolutionbandwidths (RBW) shown in FIG. 9B further illustrate the difference of alocked carrier envelope offset frequency compared to an unlocked carrierenvelope offset frequency as shown in FIG. 9A. Note that variousresolution bandwidths have been implemented only on the peak centered at60 MHz. The narrow spectral peak was obtained with a resolutionbandwidth of 1 Hz and has an intensity up to −70 dBm/Hz. The noise floorfor the 60 MHz signal is observed at around 112 dBm/Hz and was measuredwith a resolution bandwidth of 100 kHz; the noise floor is the same asseen on the peak located at 40 MHz. The transition from a resolutionbandwidth of 1 Hz to 100 kHz cannot be distinguished on FIG. 9B.

As described above, generation of a narrow bandwidth beat signal issubstantially enhanced by the use of a low dispersion oscillator cavityin conjunction with the use of a bandpass filter. This concept isfurther illustrated in FIG. 10A. A fiber oscillator 1000 comprises arange of intra-cavity fibers 1001, 1002 (only two are shown forsimplicity) with different dispersion characteristics.

Additional cavity elements comprise an element 1003 allowing for pumpinjection and also output coupling. Such an element can for examplecomprise a fiber coupler for directing pump light into the cavity aswell as an additional coupler for directing an output signal out of thecavity and to the other system components.

Saturable absorber element 1004 can be used to initiate passivemodelocking. An element 1005 comprises a bandpass filter. An element1006 can be an element allowing for polarization control, and cancomprise a polarizer as well as two rotatable waveplates, where thepolarizer is located closer to a reflector 1007.

Both fiber pigtailed cavity components as well as bulk optic cavitycomponents can be used in the cavity 1000. Also, although the cavity1000 is shown as a Fabry-Perot cavity, a ring-cavity design is alsopossible. Preferably a bandpass filter is included with an adjustment ofthe total absolute cavity dispersion smaller than approximately (10,000fs²/m)×L, where L is the intra-cavity fiber length. In variousembodiments, for example the dispersion may be about 20,000 fs² or less,about 10,000 fs² or less, or about 5,000 fs² or less, although theranges should not be limited to these.

The noise of such fiber oscillators for frequency-metrology cangenerally be reduced or minimized by the implementation of low noisediode pump lasers. Such low noise diode pump lasers are characterized bylow residual intensity noise (RIN). High performance sources can delivera RIN noise as low as approximately −150 dB/Hz and lower. In comparison,a standard grating stabilized pump source can produce a RIN noise ofapproximately −130 dB/Hz. Such pump sources have been developed forforward pumped fiber Raman amplifiers and can substantially enhance theperformance of fiber based frequency standards. Note that such low RINnoise pump sources are not single-mode, however, due to a largefrequency spacing of the modes in these pump sources being greater thanapproximately 10 GHz. Any beating between the modes has substantially noadverse effect on the stability of an Er-based laser, because of thelong Er life-time (10 milliseconds), which averages such high frequencymodulations.

FIG. 10B illustrates another embodiment of a cavity 1008 where insteadof fibers with different dispersion characteristics, a bulk dispersioncompensating element can be included inside a dispersion-compensatedfiber laser suitable for precision metrology. The cavity 1008 comprisessimilar elements 1001, 1003, 1004, 1005, 1006, and 1007 as the cavity1000 described above in reference to FIG. 10A. The cavity 1008 furthercomprises an additional dispersion compensation element 1009 such as agrating or a prism pair. Additionally, one or more Gires-Tournoisinterferometers or dispersive mirrors can also be used to achievedispersion compensation. Other configurations are also possible.

Both the Gires-Tournois interferometer and the dispersive mirror can beused instead of the reflector element 1007. The use of the dispersivemirrors and Gires-Tournois interferometers becomes practical forintra-cavity fiber lengths that are shorter than approximately 10 cm.Hence, such systems can be used for fiber or generally waveguide-basedoptical comb sources operating with pulses having repetition rates about1 GHz and higher. At repetition rates of about 1 GHz, the use ofdispersion compensated waveguide cavities for frequency comb sources isnot needed, because of the reduced dispersion of such short cavitiescompared to a cavity using a waveguide length on the order ofapproximately 2 m. Hence high repetition rate waveguide-based frequencycomb sources as shown in FIG. 10B may allow the elimination of thedispersion compensating element (such as the dispersive mirrors,Gires-Tournois interferometers and the element 1009) altogether. Asemphasized herein, particularly compact oscillator implementations arefurther enabled by the replacement of the reflector element 1007 with awaveguide grating.

It will be appreciated that the order of the various cavity elements inFIGS. 10A and 10B are exemplary, can be modified. As an example, thecavity elements 1005 (filter) and 1006 (polarization control) arearranged differently in FIGS. 10A and 10B but collectively operatesimilarly.

FIG. 11 illustrates a generic waveguide-based supercontinuum source 1100comprising polarization-maintaining elements. For industrialapplications of supercontinuum sources and waveguide-based frequencycombs, system designs based on polarization-maintaining elements arepreferable. The supercontinuum source system 1100 comprises anoscillator 1101 with a single-polarization output. The oscillator 1101can be based, for example, on designs described above in reference toFIGS. 1C, 4, 8A, and 8B. Alternatively, environmentally stable cavitydesigns may be based on combinations of polarization-maintaining andnon-polarization-maintaining fibers (or non-polarization-maintainingfibers only) in conjunction with Faraday rotators. Such designs aredescribed in greater detail in the U.S. Pat. No. 5,689,519 to Fermann etal, which is hereby incorporated herein by reference in its entirety.

The oscillator output is then preferably injected into apolarization-maintaining fiber amplifier 1102. The amplifier 1102 canalso be constructed from a non-polarization-maintaining fiber inconjunction with a Faraday rotator mirror. For high output poweroscillators the amplifier 1102 may be omitted. Preferably, however, theamplifier 1102 is operated in a nonlinear regime, such that theamplified pulses are subjected to substantial levels of self-phasemodulation, producing appreciable spectral broadening in the amplifieroutput. Therefore, because of the large levels of self-phase modulationtypically incurred in the pulse compressing fiber amplifier,polarization-maintaining fiber amplifiers are preferred forenvironmental stability.

The output from the amplifier 1102 can further be compressed by acompressor element 1105. The compressor element 1105 can comprise, byway of example, a holey fiber, a photonic band gap fiber or aconventional fiber. The compressor element 1105 can also comprise bulkdiffraction gratings, fiber gratings or periodically poled second-ordernonlinear elements such as a chirped PPLN. The chirped PPLN combinespulse compression with second-harmonic generation, which is used tochange the center wavelength of the supercontinuum source. Otherapproaches for providing compression are also possible.

As an example of a chirped PPLN in conjunction with an Er-fiber lasersystem, a supercontinuum source centered near 800 nm can be generated,in direct competition with conventional Ti:sapphire based systems. Thecompressor can generate pulses with a width of less than approximately200 fs, preferably less than approximately 100 fs and most preferablyless than approximately 70 fs to preserve coherence in supercontinuumgeneration. The compressor preferably also preserves polarization.

For the case of a chirped PPLN compressor, polarization preservation issubstantially automatic. When using a photonic bandgap fiber for pulsecompression, the use of non-centro-symmetric fiber designs is preferred.

As shown in FIG. 11, at least one highly nonlinear fiber 1103 isinserted after the compressor stage 1105 to generate the supercontinuum.The fiber 1103 is preferably also polarization-maintaining, but sinceonly very short lengths (approximately 5 mm-25 cm) of supercontinuumfiber are used in certain designs, non-polarization-maintaining highlynonlinear fibers are also acceptable. The highly nonlinear fibers can beconstructed from by way of example, holey fibers, gas filled photonicbandgap fibers, as well as conventional optical fibers. An output 1104from the system 1100 can then be used for absolute phase locking of theoscillator 1101.

FIG. 12 illustrates one embodiment of a highly integrated frequency combsource 1200. Preferably such a system is substantially fiber-based,having mainly fiber components. Such a highly integrated system is morepreferable for mass application of this technology than systems that usemany separate bulk components (such as bulk doubling crystals) for thef-2f interferometer. Compact size and ruggedness are some examples ofadvantages of such systems.

The highly integrated system 1200 comprises an oscillator 1201, anisolator 1202, an amplifier 1203, and a highly nonlinear fiber 1204 thatperform similar roles as those described above in reference to FIG. 8A(801, 802, 803, and 805). The isolator 1202 and/or amplifier 1203 may beexcluded from this system 1200 as discussed above. The nonlinear fiber1204 may also be implemented by other elements as well. The system 1200further comprises one or more additional coupling fibers 1205 that arespliced to the highly nonlinear fiber 1204 so as to provide approximatemode-matching to a nonlinear waveguide 1206. The coupling fiber 1205enables coupling of the supercontinuum output from the highly nonlinearfiber 1204 to the nonlinear waveguide 1206.

The nonlinear waveguide 1206 is preferably single-mode for both red andblue ends of the supercontinuum, and can for example be based on aLiNbO₃ waveguide. A periodically poled grating section 1207 can befurther included in the nonlinear waveguide 1206 to frequency-double thered-part of the supercontinuum thereby enabling interference with thenon-doubled (blue) part of the supercontinuum. In this inlineinterferometer design, one arm of the interferometer includes thefrequency multiplier 1207 that produced the frequency doubledcomponents. The frequency doubled components interfere with thenon-frequency doubled components in the one arm thereby producing beatfrequencies in the one arm. The position of grating 1207 withinnonlinear waveguide 1206 and the length of coupling fiber 1205 can beselected to facilitate temporal overlap of the doubled and non-doubledparts of the supercontinuum. As described above, preferably, oppositegroup velocities (e.g., dispersions) at least partially offset phasedelays for the doubled and non-doubled frequency components of thecontinuum.

The system 1200 further comprises a coupling fiber 1208 that couples thelight out of the waveguide 1206. An optical bandpass filter 1209 can beinserted in front of a detector 1210. The detector 1210 can be used toobserve the carrier envelope offset frequency beat signal as well as therepetition rate of the oscillator in a manner similar to that describedabove in reference to FIG. 8A. The bandpass filter 1209 can be selectedto transmit substantially only those parts of the supercontinuumspectrum that actually contribute to the beat signal.

The system 1200 further comprises a feedback line 1211 that providesfeedback(s) to control the repetition rate and the absolute phase ofoscillator 1201. The system 1200 may further comprise a broad-bandwidthcoupler 1212 inserted into the coupling fiber 1205 to extract thestabilized frequency combs to an external output 1213 for externalapplications.

Another example of an in-line fiber laser system 1300 with a feedbackloop section 1302 for phase locking f_(ceo) is shown in FIG. 13. Thisin-line system 1300 includes a laser oscillator 1304, an isolator 1306(optional), an amplifier 1308 (optional), and a highly nonlinear opticalfiber 1310. The oscillator 1304, amplifier 1308, and nonlinear medium1310 preferably substantially comprise fiber optic components, however,other types of non-fiber devices may be employed as well. For example,highly nonlinear waveguide structures may also be incorporated.

The oscillator 1304 may be mode-locked with a passive mode-lockingelement such as a saturable absorber that comprises one of a pair ofreflective optical elements that forms a resonant cavity in theoscillator. Other configurations and designs to accomplish mode-locking,however, are possible. Mode-locking the oscillator 1304 results in atrain of optical pulses with a corresponding frequency spectrum.Propagation of these pulses through the highly non-linear medium 1310yields a large spectrum of frequencies referred to above as a continuumor supercontinuum, which is preferably at least one octave.

A single arm interferometer 1312 having a single arm 1314 disposed inthe optical path of the output of the highly nonlinear fiber 1310includes a frequency doubler 1316 or generally a nonlinear crystalwaveguide in the single arm. The frequency doubler 1316 generates asecond set of frequencies as well as a corresponding second train ofoptical pulses. Other components or elements that produce a second setof frequency components (and second set of pulses) may be used in otherembodiments. For example difference frequency generation in a nonlinearcrystal may also be implemented. This second set of frequency componentsare preferably shifted in frequency with respect to the first set offrequency components, however, as described above, the optical pulsescorresponding to this second set of frequencies preferably temporallyoverlap the first set of pulses. This second set of frequency componentsmay be a multiple or a fraction of the first set of frequencycomponents.

The first and second trains of pulses interfere with each other in thesingle arm 1314 of the single arm interferometer 1312 yielding anoptical interference signal that is detected by an optical sensor 1318.Beat frequencies from the first and second sets of frequency componentsare provided in an electrical output from the optical sensor 1318.

The feedback system 1302 includes electronic filters 1320, 1322 thatisolate beat frequencies, f_(rep) and f_(rep)+f_(ceo). In the exampleshown in FIG. 13, these frequencies correspond to 52 MHz and 64 MHz,respectively. A clock 1324, such as a Rb clock with a 10 MHz oscillationfrequency, can be utilized as a reference for phase locking to the beatfrequencies. Frequency synthesizers 1326, 1328 transforms the 10 MHzoscillation reference into an useful frequency for synchronization. Inthe design shown in FIG. 13, the frequency synthesizers 1326, 1328,yield a 52 MHz and 4 MHz synch, respectively. The 52 MHz synchronizationsignal is phase locked to the f_(rep) using a first phase detector 1330that provides feedback to a piezo or other electro-mechanical transducerthat alters the cavity in the oscillator 1304 and thereby shift therepetition frequency. The beat frequency, f_(rep)+f_(ceo) passed throughthe 64 MHz filter 1322, is scaled down (here by a factor 16) andcompared to the 4 MHz synch signal by a second phase detector 1332.Phase locking of this beat frequency is accomplished by adjusting thepump power to the oscillator 1304 so as to alter the f_(ceo)accordingly. Loop filters 1334, 1336 are included in the feedback pathsto the oscillator. Other designs, configurations, and arrangements canbe implemented as well.

The above description of the preferred embodiments has been given by wayof example. From the disclosure given, those skilled in the art will notonly understand the present invention and its attendant advantages, butwill also find apparent various changes and modifications to thestructures and methods disclosed. It is sought, therefore, to cover allsuch changes and modifications as fall within the spirit and scope ofthe invention, as defined by the appended claims, and equivalentsthereof.

1. A frequency comb source for producing a frequency comb, saidfrequency comb source comprising: a mode-locked fiber oscillatorcomprising a resonant Fabry-Perot optical cavity having a cavity length,L, said mode-locked fiber oscillator outputting optical pulses andcorresponding frequency components separated by a frequency spacing,f_(rep) and offset from a reference frequency by a frequency offset,f_(ceo); a non-linear optical element positioned to receive said opticalpulses, said non-linear optical element having sufficient opticalnon-linearity to generate additional frequency components that togetherwith said plurality of frequency components output by said mode-lockedoscillator form a first set of frequencies separated by said frequencyspacing, f_(rep) and offset from said reference frequency by saidfrequency offset, L_(ceo); an interferometer optically coupled toreceive said first set of frequencies, said interferometer comprising afrequency shifter that receives said first set of frequencies and thatsuperimposes a second set of frequencies on said first set offrequencies received by said frequency shifter, said second set offrequencies interfering with said first set of frequencies to producebeat frequencies; and an optical detector optically receiving said beatfrequencies, said optical detector having an output for outputting saidbeat frequencies.
 2. The frequency comb source of claim 1, furthercomprising a feedback system having an input for receiving said beatfrequencies, said feedback system in communication with said mode-lockedfiber oscillator so as to control the offset frequency, f_(ceo) based onsaid beat frequencies.
 3. The frequency comb source of claim 2, whereinsaid feedback system has an input for a reference signal and phasedetection electronics to compare at least one of said beat frequencieswith the reference signal.
 4. The frequency comb source of claim 1,wherein said non-linear optical element comprises a non-linear opticalfiber.
 5. The frequency comb source of claim 1, wherein said frequencyshifter comprises a frequency multiplier.
 6. The frequency comb sourceof claim 1, wherein said first and second sets of frequencies areinterfered over substantially the same optical path in saidinterferometer.
 7. The frequency comb source of claim 1, wherein saidinterferometer comprises a group delay compensator that compensates fora difference in optical path between said first and second sets offrequencies interfering in said interferometer, said group delaycompensator reducing said optical path difference to substantially zero.8. The frequency comb source of claim 1, wherein said interferometercomprises an optical path having first and second portions with oppositefirst and second dispersion values, said first dispersion valueproviding relatively lower group velocity for a first train of opticalpulses corresponding to said first set of frequencies and said seconddispersion value providing relatively higher group velocity for a secondpulse train corresponding to said second set of frequencies such thatsaid first and second pulse trains substantially overlap in time andthereby interfere.
 9. The frequency comb source of claim 8, wherein saidgroup delay compensator element is selected from the group consisting ofa segment of dispersive optical fiber and a planar waveguide elementhaving a group delay dispersion different for the first and second setsof frequencies.
 10. The frequency comb source of claim 1, wherein saidnon-linear optical element has positive dispersion to lower the noiserelated to Raman processes in said non-linear optical element.
 11. Thefrequency comb source of claim 10, wherein said non-linear opticalelement comprises a segment of non-linear fiber comprising non-linearbismuth-oxide optical glass.
 12. A method of producing a frequency comb,said method comprising: substantially mode-locking longitudinal modes ofa fiber laser cavity so as to produce laser pulses; propagating saidlaser pulses through a non-linear optical element so as to produce afirst plurality of frequency components offset from a referencefrequency by frequency offset, f_(ceo); propagating said laser pulsesalong an optical path that leads to an optical detector; generating asecond plurality of frequency components from said first plurality offrequency components and propagating said first and second plurality offrequency components on said optical path leading to said opticaldetector; interfering said first plurality of optical components withsaid second set of optical components along said optical path to saidoptical detector so as to produce at least one beat frequency; and usingsaid at least one beat frequency to stabilize said offset frequency,f_(ceo).
 13. The method of claim 12 further comprising varying thedispersion along a portion of said optical path such that a first set ofpulses corresponding to said first plurality of frequencies propagatesat a first group velocity and a second set of pulses corresponding tosaid second set of frequencies propagates at a second group velocity sothat said first set of pulses substantially overlaps said second set ofpulses in time at said detector.
 14. A frequency comb source comprising:a mode-locked fiber oscillator comprising an optical fiber and a pair ofreflective optical elements that form an optical cavity that supports aplurality of optical modes, said mode-locked fiber oscillatormode-locking said optical modes to produce optical pulses and frequencycomponents having a frequency spacing, f_(rep), and offset from areference frequency by a frequency offset, f_(ceo); a substantiallynon-linear optical element disposed to receive said optical pulses, saidsubstantially non-linear optical element having sufficient opticalnon-linearity to generate additional frequency components that togetherwith said frequency components output from said mode-locked fiberoscillator form a first plurality of frequency components spaced by saidfrequency spacing, f_(rep), and offset from said reference frequency bysaid frequency offset, f_(ceo); an interferometer that interferes asecond plurality of optical frequency components with said firstplurality of frequency components thereby producing beat frequencies;and an optical detector optically connected to said interferometer todetect said beat frequencies, said optical detector having an outputthat outputs said beat frequencies.
 15. The frequency comb source ofclaim 14, further comprising a first feedback system and a secondfeedback system each having an input for receiving said beatfrequencies, said first and second feedback systems each comprising aphase lock loop to compare said beat frequencies with one or morereference frequencies, said first electronic feedback system connectedto said mode-locked fiber oscillator to control said offset frequency,f_(ceo), based on said beat frequencies, said second feedback systemconnected to said mode-locked fiber oscillator to control saidrepetition frequency, f_(rep).
 16. The frequency comb source of claim15, further comprising a transducer that perturbs said optical fiber insaid mode-locked fiber oscillator in response to feedback from saidfirst feedback system to alter the offset frequency, f_(ceo).
 17. Thefrequency comb source of claim 15, further comprising a transducer thatperturbs said optical fiber in said mode-locked fiber oscillator inresponse to feedback from said second feedback system to alter therepetition frequency, f_(rep).
 18. The frequency comb source of claim15, further comprising an optical pump for pumping said mode-lockedfiber oscillator, said optical pump selectively adjusting said pumpingin response to feedback from said feedback system to control the offsetfrequency, f_(ceo).
 19. The frequency comb source of claim 15, furthercomprising a transducer that adjusts one of said reflective opticalelements in response to feedback from said first feedback system toalter the offset frequency, f_(ceo).
 20. The frequency comb source ofclaim 15, further comprising an optical coupler for coupling output fromsaid oscillator to measure said f_(rep), said second feedback systemadjusting said oscillator based on said measured f_(rep).